The present invention relates generally to high quality video display systems and, more particularly, to a technique for improving the quality of the video presentation produced on the viewing screen of a television receiver in response to a source of text and/or graphics information such as a home computer or the like.
Standard television receivers are often used to display text and/or graphics information derived from various sources in addition to broadcast television programming signals. Such sources of text and/or graphics information typically include home computers, TV games, viewdata decoders, character generators, etc. Devices of this type normally include a memory which stores digitally encoded data representing a selected text and/or graphics video image intended for display on the viewing screen of the receiver. The contents of the memory are controlled by a control circuit such that the stored digitally encoded data may be updated from time to time to reflect desired changes in the video presentation. The stored digitally encoded data is converted by a display generator to corresponding video signals which are then modulated on an RF carrier and coupled to the antenna input terminals of the television receiver which converts the video signals to a corresponding display image. The quality of the resulting display image is therefore limited by the characteristics of the video processing circuits of the television receiver, which limitations include bandwidth restrictions and the like.
The present invention seeks to improve the quality of the display images produced in the foregoing manner through the use of a teletext type decoder which is built into the television receiver. More specifically, teletext is a generic term describing a television communications system using the vertical interval of a broadcast televison signal to transmit digitally encoded text and/or graphics information. A television receiver designed for displaying such teletext transmissions includes a teletext decoder responsive to the output of the video detector of the receiver for converting the received teletext encoded information into corresponding R, G, B video baseband signals which are used to directly drive the CRT of the receiver. Since the R, G, B video baseband signals developed by the teletext decoder directly drive the CRT, the resulting display image is not compromised or limited by the restricted bandwidth video processing circuits of the receiver with the result that an extremely high quality video display is produced on the viewing screen of the receiver. The ability of television receivers having built-in teletext capability to product high quality video displays is advantageously utilized in accordance with the present invention by converting the digitally encoded data stored in the memory of an external source of text and/or graphics information to a corresponding teletext endoded signal which may be processed by the television receiver. As a consequence, the stored data results in the generation of corresponding baseband video signals at the output of the teletext decoder for directly driving the CRT of the television receiver to produce a high quality video image, such being contrasted with the prior art technique of coupling RF modulated video signals representing the stored data to the antenna input terminals of the receiver for processing by the bandlimited video processing circuits thereof.
In another aspect of the invention, compatibility is maintained with standard television receivers (i.e. receivers not including built-in teletext decoders) by transmitting standard video signals during the active portions of the television signal corresponding to the teletext encoded data transmitted during the vertical intervals thereof. Thus, a standard television receiver may respond to the standard video signals for displaying the text and/or graphics information, although in a relatively low quality form, while a teletext equipped receiver may respond to the teletext encoded signals for producing the same display but at a higher quality level.